PURDUE (US) — When it comes to mealtimes, chimpanzees eat smart, and may give serious thought to the quality and timing of their food.

“There is an association between the time of day primates eat certain resources and the nutritional quality of those resources, suggesting consumption may track nutrient content,” says Bryce Carlson, an assistant professor of anthropology at Purdue University who studies primate ecology and nutrition in human evolution.

“We can’t say for sure if chimpanzees are consciously selecting the leaves when nutritional content is greatest, but this correlation presents an intriguing hypothesis to explain feeding behavior in this primate species and mechanisms for ingestive behavior in general.”

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Chimpanzees, whose diet is composed of fruit, leaves, plant stalks, roots, insects, and other vertebrate animals, frequently consume various leaves at the end of the day.

Other researchers have proposed the animals prefer eating leaves at that time to feel full and facilitate greater nutrient absorption overnight, or that this daily eating pattern results from social dynamics, where chimpanzees typically spend late afternoons in smaller foraging groups on the ground where these leaves are found.

“But we know there is a correlation between nutritional quality and daily feeding patterns for other animals, such as domesticated sheep,” Carlson says. “So we wanted to take a closer look at chimpanzees by comparing the primates’ feeding habits to the nutritional composition of these leaves throughout the day.”

For the study, published in the American Journal of Primatology, researchers collected data regarding the chimpanzees and two species of saplings, Pterygota mildbraedii and Celtis africana, from Ngogo in Uganda’s Kibale National Park. The Ngogo chimpanzee community is the largest observed in the world—more than 180 animals—and has been actively studied since 1995.

Daily feeding observations from 2002-2011, made primarily during the dry season months of June through August, of 41 adult male chimpanzees were analyzed for eating patterns. These were compared to nutrition samples from Pterygota mildbraedii and Celtis africana. Leaf samples were taken from different saplings and at various feeding times during the day.

Sweeter in the afternoon

Pterygota mildbraedii is a very large tree, common throughout the Ngogo chimpanzee habitat. The chimpanzees, however, eat young leaves of the saplings found near the forest floor. This study found that the leaves’ hemicelluloses—a more digestible fiber—and nonstructural carbohydrates—simple sugars and starch—increased 15 percent to 100 percent, respectively, from morning to evening.

Cellulose and lignin, which make the leaves more difficult to digest, also decreased by day’s end. Celtis africana is a smaller tree than Pterygota, the saplings of which contain many thin branches and small leaves. The sugars in this plant’s leaves were found to double from morning to late afternoon.

“If these sugars or total non-structural carbohydrates are increasing, then the leaves are returning more calories late in the day,” Carlson says. “At this time, they may taste sweeter, and the chimpanzees may then learn and adjust their feeding behavior accordingly. We know they use vision, texture, taste, and smell to gauge when to eat fruit, so it’s understandable to think they may do the same with leaves.”

Carlson’s research will continue to focus on diet for wild chimpanzees and human ancestors.

“Questions about what humans are eating today and why are important as our growing world population increasingly struggles with malnutrition tightly linked with quality of life, morbidity and mortality,” he says. “Evolution for any species is related to, and even driven by, food availability and quality, so the more we learn about our long history with food, the better able we are to make individual and population level recommendations for consumer behavior today and years to come.”

Researchers from Emory University, Hunter College of The City University of New York, and the University of Michigan contributed to the study, which was funded by the National Science Foundation and L.S.B. Leakey Foundation.